title: Regulation of RNA-dependent RNA polymerase 1 and isochorismate synthase gene expression in Arabidopsis.
abstract: BACKGROUND
  RNA-dependent RNA polymerases (RDRs) function in anti-viral silencing in Arabidopsis thaliana and other plants. Salicylic acid (SA), an important defensive signal, increases RDR1 gene expression, suggesting that RDR1 contributes to SA-induced virus resistance. In Nicotiana attenuata RDR1 also regulates plant-insect interactions and is induced by another important signal, jasmonic acid (JA). Despite its importance in defense RDR1 regulation has not been investigated in detail.
  
  
  RESULTS
  In Arabidopsis, SA-induced RDR1 expression was dependent on 'NON-EXPRESSER OF PATHOGENESIS-RELATED GENES 1', indicating regulation involves the same mechanism controlling many other SA- defense-related genes, including pathogenesis-related 1 (PR1). Isochorismate synthase 1 (ICS1) is required for SA biosynthesis. In defensive signal transduction RDR1 lies downstream of ICS1. However, supplying exogenous SA to ics1-mutant plants did not induce RDR1 or PR1 expression to the same extent as seen in wild type plants. Analysing ICS1 gene expression using transgenic plants expressing ICS1 promoter:reporter gene (β-glucuronidase) constructs and by measuring steady-state ICS1 transcript levels showed that SA positively regulates ICS1. In contrast, ICS2, which is expressed at lower levels than ICS1, is unaffected by SA. The wound-response hormone JA affects expression of Arabidopsis RDR1 but jasmonate-induced expression is independent of CORONATINE-INSENSITIVE 1, which conditions expression of many other JA-responsive genes. Transiently increased RDR1 expression following tobacco mosaic virus inoculation was due to wounding and was not a direct effect of infection. RDR1 gene expression was induced by ethylene and by abscisic acid (an important regulator of drought resistance). However, rdr1-mutant plants showed normal responses to drought.
  
  
  CONCLUSIONS
  RDR1 is regulated by a much broader range of phytohormones than previously thought, indicating that it plays roles beyond those already suggested in virus resistance and plant-insect interactions. SA positively regulates ICS1.
authors.affiliation: Department of Plant Sciences, University of Cambridge, Cambridge, United Kingdom.
authors.forename: Lydia J R
authors.initials: LJ
authors.lastname: Hunter
authors.forename: Jack H
authors.initials: JH
authors.lastname: Westwood
authors.forename: Geraldine
authors.initials: G
authors.lastname: Heath
authors.forename: Keith
authors.initials: K
authors.lastname: Macaulay
authors.forename: Alison G
authors.initials: AG
authors.lastname: Smith
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Macfarlane
authors.forename: Peter
authors.initials: P
authors.lastname: Palukaitis
authors.forename: John P
authors.initials: JP
authors.lastname: Carr
mesh_terms: D000040:Abscisic Acid / Q000494:pharmacology; D017360:Arabidopsis / Q000201:enzymology*; D001483:Base Sequence; D017931:DNA Primers; D005030:Ethylenes / Q000494:pharmacology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D019751:Intramolecular Transferases / Q000235:genetics*; D030821:Plants, Genetically Modified; D012324:RNA-Dependent RNA Polymerase / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D017931:DNA Primers; D005030:Ethylenes; D000040:Abscisic Acid; C036216:ethylene; D012324:RNA-Dependent RNA Polymerase; D019751:Intramolecular Transferases; C058486:isochorismate synthase